Reduction of noise



E0, 21939. w. R. KocH REDUCTION OF NOISE Filed March 3l, 1937 3 Sheets-Sheet 1 II'IL CHER/EF MOD.

.IIIIIH HUD/O WMP.

VOL 757 L/M/TLW 1 ssuB MHH/ER AMP NIT

.DET

I/V-n @Zd E. Koch,

@et E0, E939., w. R. .KOCH 2,275,237@

REDUCTION OF NOISE Filed Marchl, 1937 3 Sheets-Sheet 3 Patented Get. l0, 193g REDUCTIGN F NOISE Winield R.. Koch, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 31, 1937, Serial No. 133,962

8 Claims.

This invention relates to the reduction of noise, such as that produced in radio receivers and like apparatus by impulses extraneous to the desired signal, and has for its principal object the provision of an improved system and method of operation whereby the undesired eifects of such noise impulses may be obviated without modification or distortion of the received signal.

Various types of noise reduction systems have been proposed in the past. Many of these systems, however, have not been altogether satisfactory in practice for the reason that they eliminate not only the undesired noise impulses but also modify or distort the desired signal impulses. In accordance with one form of the present invention, this dimculty is avoided by the provision of a signal transmitting and receiving system wherein two signal portions are transmitted as side bands of a doubly modulated carrier. This system functions (l) to separate the signal into two signal portions which are dephased with respect to one another and are utilized to modulate different sub-carriers, (2) to combine these modulated sub-carriers with a main carrier by which the similar but out-ofphase signals are transmitted, (3) to receive these similar but dephased signals with the same noise impulse superimposed at different points on their respective waves, (1i) to bring the two signals into phase with one another thus dephasng their superimposed noise impulses, and (5) to utilize the level of one of the signals to limit the level at which the other signal is supplied to a utilization circuit such as a succeeding audio frequency amplier or the like.

In other forms of the invention, the similar but dephased signals may be transmitted, respectively, as vertically polarized waves and as horizontally polarized waves, with slightly different frequency carriers, or the like. An important feature of the invention is a cross limiting device which functions to compare the audio frequency levels of the two signals and to limit the transmitted signal 'to the level of the other signal irrespective of the presence of noise impulses superimposed on the two signals.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope is indicated by the appended claims.

Referring to the drawings:

Fig. 1 illustrates a system wherein separate signal portions are transmitted as side bands of a doubly modulated carrier,

Fig. 2 illustrates a receiver adapted to be utilized in connection with the transmitter of Fig. 1,

Fig. 3 is a wiring diagram of the limiter device of the receiver of Fig. 2,

Fig. 4 illustrates a receiving and transmitting 5 system wherein the separate signal portions are transmitted respectively as horizontally and vertically polarized waves,

Fig. 5 illustrates a transmitting and receiving system wherein the different signal portions are 10 transmitted with different carrier frequencies,

Fig. 6 is a wiring diagram of a modified cross limiter which permits transmission of the different signal portions, and

Fig. 7 is a wiring diagram of a delay network 1li which is one suitable form of means for controlling the phase relation between the different signal portions.

The transmitting system of Fig. l includes an audio frequency source, illustrated as an ampli- 20 fier I9, from which two portions of the signal are supplied through separate circuits to a modulator I I. One of these circuits includes a modulator I2 and a sub-carrier source I3, the output of which is modulated and phased as indicated 25 by the curve It. The other `cf these circuits includes a time delay network I5 or the equivalent, a modulator I6 and a sub-carrier source I'I, the output of which is modulated and phased as indicated by the curve I8. The combined output a0 of these two circuits is supplied from the modulator Il to the transmitter antenna 2B through carrier source 20, the output of which is modulated by the combined outputs of the two circuits.

It will readily be understood that the time 35 delay device I5 may assume various forms, that the sub-carriers are of different frequencies and that the main carrier is of a different and higher frequency than those of the sub-carriers.

The receiver of Fig. 2 includes an antenna 2l 40 and a first detector 22 which eliminates from the signal the main carrier originated at the source 2U. From the detector 22, one signal portion is supplied through a second sub-carrier amplifier 2A, a detector 25 and an audio frequency ampli- 45 fier 26 to a voltage limiter 23. The other signal portion is supplied to the voltage limiten` 23 through a first sub-carrier amplier 2i, a detector 28, a delay network 29 and an audio amplifier 30.

The audio signal is indicated at 3l and the superimposed noise impulse at 32. It will be noted that, in the outputs oi the detectors 25 and 28, the noise impulses 32 vare in phase while the audio impulses are displaced in phase and that, in the :i5i

Ill

input circuits of the audio amplifiers 26 and 30, the audio impulses 3| are in phase while the noise impulses 32 are displaced in phase with respect to one another. It therefore results that the noise impulses 32 are superimposed on different parts of the otherwise similar audio impulses delivered to the limiter 23.

The circuit connections of the limiter 23 (Fig. 2) are shown in Fig. 3. This circuit includes a rectifier 33 which has its cathode connected to ground through a resistor 34 and its anode connected to ground through resistors 35 and 36 and a direct current potential source 31. With these connections, there is produced in the resistor 34 an audio frequency potential which is a measure of the level of one of the signal portions which is delivered to it through the coupling 39--46 and the electron discharge device 4I. There is likewise produced in the resistor 36 an audio frequency potential which is a measure of the level of the other of the signal portions which is delivered to it through the coupling capacitor 42.

So long as the two signal portions are equal in level and free from noise impulses, the cathode and anode of the rectifier 33 are maintained at substantially the same potential, no current flows throu-gh the rectifier 33 and signal impulses are supplied from the amplifier 26 to an amplifier 43 as indicated at 26. If a noise impulse is superimposed on the signal portion delivered from the amplifier 33 to the resistor 34, the cathode of the rectifier 33 is made more positive than the rectifier anode, the rectifier still transmits no current and the signal is supplied from the amplifier 26 to the amplifier 43 without distortion. If a noise impulse is superimposed on the signal delivered by the amplifier 26, the rectifier anode becomes more positive than its cooperating cathode, the rectier transmits current and the signal from the amplifier 26 is delivered to the amplifier 43 at a level which is measured by the audio frequency drop of the resistor 34. Otherwise stated, the rectier functions through the audio frequency drop of resistor 34 to shave the noise impulse off the transmitted signal which is delivered undistorted to the amplifier 43.

In order to ensure that this result is accomplished accurately, there is provided in the output circuit of the amplifier 26 a potential source 31 which compensates for the zero signal drop in the plate resistor 34 of the device 4l. It will be understood that this type of limiter action is made possible by the fact that the noise impulse 32 is superimposed at different points of the two signal wave portions.

The system of Fig. 4 is similar to that of Fig. 1 with the exception that it includes means for transmitting one of the signal portions as horizontally polarized waves and the other signal portion as vertically polarized waves. To this end, an oscillation generator 44 is arranged to supply a carrier to the modulators l2 and I6 and the two signals are radiated from different antennas 45 and 46. These differently polarized signals are received respectively by the antenna 41 andthe antenna 48 and are supplied from these antennas to detectors 49 and 50 in which they are combined with heterodyne oscillations derived from a generator 5|. From the detectors 43 and 5l), the signals are supplied respectively through the intermediate frequency amplifier 52 to the detector 23 and through the intermediate amplifier 53 to the detector 25.

Since the systems of Figs. l and 4 differ only in that the two signals modulate different subcarriers in the one case and are in the form of two differently polarized in the other case, they operate in substantially the same manner to eliminate the superimposed noise from the signal.

The system of Fig. 5 is similar to those of Figs. 1 and 4 in some respects but is distinguished therefrom in that the two portions of the signals are transmitted by separate carriers of different frequency, Thus one portion of the signal is delivered from the audio amplifier l0 through the modulated amplifier 54 and a filter 51 to the amplifier 58 and another portion df the signal is delivered to the amplifier 58 through a time delay network l5, a modulated amplifier 59 and a filter 60. Associated with the amplifier 54 is a carrier source 6| of one frequency Vand similarly associated with the amplifier 59 is a carrier source 62 of a different frequency. Since the lters 51 and 66 are tuned respectively to the frequencies of the generators 6i and 62, the two signals are transmitted from the antenna 63 by means of different carriers.

These two signals of different carrier frequency are received by the antenna 64 and delivered to a detector 65 which is associated with a heterodyne frequency source and which delivers the two signals respectively through an intermediate frequency amplifier 61 to the detector 25 and through an intermediate frequency amplifier 68 to the detector 28.

The operation of this system is not substantially different from that of the systems of Figs. 1 and 4, the distinction in each case being in the means provided to segregate the signals. Thus, separation of the signals is effected in the system of Fig. 1 by the utilization of different subcarriers, in the system of Fig. 4 by the utilization of different polarization, and in the system of Fig. 5 by different carrier frequencies. These various features combined with the time delay networks ensure that the signals, in each system, arrive at the limiter 23 with the noise impulse 32 superimposed at different points of their respective waves so that one of the signals may measure the level at which the other is transmitted.

The limiter of Fig. 6 is distinguished from that of Fig. 3 by the fact that it permits both portions of the signal to be transmitted to an output circuit 10 without distortion due to the removal of a superimposed noise impulse. Thus, the noise-free signal output from the audio arnplifier 26 is delivered to the amplifier 43 as explained in connection with Fig. 3, a stage 1| being included in the circuit to reverse the signal wave polarity. Similarly, the devices 33A, 34A, 35A, 31A and 4|A function to remove a noise impulse from the signal delivered from. the amplifier 36 to an amplifier 43A. It will be noted that the amplifiers 43 and 43A are connected in pushpull. This connection, of course, has the well known advantage that certain undesired harmonies are cancelled.

More specifically stated, the battery 31 functions to neutralize the zero signal potential of the resistor 34, the battery 31A functions to neutralize the zero signal potential of the resistor 34A, the signal output of the amplifier 26 as compared with the signal output of the amplifier 36 by the device 33 is supplied through the phase reversing tube 1I to the input circuit of the pushpull connected output tube 43, and the signal output of the amplifier 3@ as compared with the signal output of the amplifier 26 by the device 33A is supplied to the input circuit of the output tube 43A. Since the noise components of the two signals are out of phase with one another, it necessarily follows that each signal operates to eliminate this component from the other signal and there is derived at the output 10 of the pushpull amplier 43--43A a signal which is free from noise.

Fig. '7 illustrates a time delay network which is suitable for producing the desired spacing between a noise impulse superimposed on the different signals so that, at any instant, one of the signals may produce an audio frequency potential Which functions to measure the level at which the other signalis transmitted. This result can not be achieved if the noise impulse is superimposed at the same point of the two signal waves. Obviously, this time delay may be produced by other means, such as a telegraphone, an acoustic path of adjustable length or the like.

I claim as my invention:

1. The noise reducing method which includes separating a signal into two parts, delaying one of said signal parts, transmitting said signal parts, delaying the other of said signal parts, and utilizing an audio frequency potential dependent on one of said signal parts to regulate the amplitude of the other of said signal parts.

2. The noise reducing method which includes separating a signal into two parts, delaying one of said signal parts, utilizing said signal parts to modulate sub-carriers of different frequencies, transmitting said signal parts, delaying the other of said signal parts, and utilizing an audio frequency potential dependent on one of said signal parts to regulate the amplitude of the other of said signal parts.

3. The noise reducing method which includes separating a signal into two parts, delaying one of said signal parts, transmitting said signal parts respectively as horizontally and vertically polarized waves, delaying the other of said signal parts, and utilizing an audio frequency potential dependent on one of said signal parts to regulate the amplitude of the other of said signal parts.

4. The noise reducing method which includes separating a signal into tWo similar parts, delaying one of said signal parts, transmitting said signal parts by means of carriers of diierent frequencies, delaying the other of said signal parts,

and utilizing an audio frequency potential de pendent on one of said signal parts to regulate the amplitude of the other of said signal parts.

5. The noise reducing method which includes separating a signal into similar parts, delaying one of said signal parts, transmitting said signal parts, similarly delaying the other of said signal parts and utilizing one of said signal parts to produce an alternating current potential which limits the other of said signal parts to its original wave shape.

6. A noise reducing system including means for separating a signal into similar parts which are of the same frequency, means for delaying one of said signal parts, means for transmitting said signal parts through a medium wherein they are likely to be combined with an undesired component, means for similarly delaying the other of said signal parts, means for deriving from one of said signal parts an alternating current potential, and means for utilizing said derived potential to eliminate said undesired component from the other of said signal parts.

7. A noise reducing system including means for separating a signal into similar parts which are of the same frequency, means for delaying one of said signal parts, means for transmitting said signal parts through a medium wherein they are likely to be combined with an undesired component, means for similarly delaying the other of said signal parts, and means including a rectier provided With a cathode and anode subjected respectively to potentials dependent on the instantaneous amplitude of said signal parts for excluding said undesired component from one of said parts.

8. The combination of a source of similar but dephased audio frequency signals each including an undesired component, means for bringing said signals into phase with one another, and means including an electron discharge device provided with a cathode and anode subjected respectively to audio frequency potentials proportional to said signals for excluding said undesired component from one of said signals, one of said audio frequency signals only being applied to the cathode and another only being applied to the anode.

WINFIELD R. KOCH. 

